A typical ink supply system for ink jet printing apparatus is described in the Kruspe, et al. patent case, Ser. No. 308,365, filed Nov. 21, 1972, having H. R. Kruspe, et al, as inventors, and entitled "Pressurizing System for Ink Jet Printing Apparatus." Ink circulating in a system of this nature has been subject to fungus and bacterial deterioration during operation of the system, while the ink usually contains a biocide, the surface areas not continually exposed to the biocide - containing ink tend to support fungus and/or bacterial growth thus clogging filters, etc. as growth debris is carried by the ink in the system.